The present invention relates to a procedure for measuring the flow velocity of a suspension flow. More particularly, the invention relates to a method of measuring the flow velocity of a suspension flow by utilizing ultrasonics.
Ultrasonic transducers are mounted on the surface of the pipe or equivalent through which the suspension flows. The transducers are spaced a predetermined distance from each other in the direction of flow and transmit an ultrasonic pulse through the pipe or equivalent into the suspension flow. The ultrasonic pulse passing through the suspension is received after a time determined by the measuring depth. The low frequency signal contained in the received pulses, caused by fibres or other particles in the suspension, is amplitude-detected and the signals derived from the transducers are compared by correlation to determine the flow velocity of the suspension.
The invention also relates to apparatus for measuring the flow velocity of a suspension, in particular, a paper pulp suspension, by ultrasonic echo pulses.
Precision flow velocity meters known in the art may be divided into measuring instruments of four different types. These are the magnetic meter, the ultrasonic meter, the vortex meter and the horizontal turbine meter.
The magnetic meter is insensitive to the velocity profile and, in addition, the pressure drop is nearly nil. The magnetic meter has an accuracy of .+-.0.5% and 0.5% of full scale deflection. Furthermore, the magnetic meter is awkward to install and is expensive.
The ultrasonic meter is sensitive to the velocity profile, but the pressure drop caused by said meter is nearly nil. The accuracy of the ultrasonic meter is .+-.0.1%. The ultrasonic meter has to be recalibrated when a suspension to be measured flows through different pipes.
The vortex meter is not operative at all with low flow velocities and causes a considerable pressure drop. The accuracy of the vortex meter is .+-.0.5%. Furthermore, the vortex meter is awkward to install.
The horizontal turbine meter causes a considerable pressure drop and its range of operation is comparatively narrow. The accuracy of the horizontal turbine meter is .+-.0.25%. In addition, the horizontal turbine meter is awkward to install.
The state of art constituting the starting point of the invention consists of the following references. "Automaatiopaivat (Automation Days)", 1981, III, p. 425, "Ultraaanen ja ristikorrelaatiotekniikan Ka yttoo n perustuva virtausmittari (A Flow Meter Based on the Use of Ultrasonics and of Cross-Correlation Technique)", U.S. Pat. No. 3,762,221, and German application print No. 2,321,832.